The present invention relates to a method for matching the bit rate of a bit stream to be transmitted in a communication system, particularly a mobile radio system, and further relates to a corresponding communication device.
The area of mobile radio technology is undergoing rapid development. Right now work is underway on standardization of what is known as the UMTS (“Universal Mobile Telecommunication System”) mobile radio standard for third-generation mobile radio terminals.
Such standard involves performing rate matching on the transmitter side in order to match the bit rate of the bit stream to be transmitted to the possible transmission rate, in which bits are either removed from the bit stream or multiplexed in the bit stream; in particular, duplicated. The removal of bits is referred to as puncturing and the multiplexing of bits as “repetition”.
A possible layout of the transmission path for which this type of bit rate matching is envisaged is shown in an example in FIG. 1.
A data stream including a number of data or transport blocks is first expanded by a device by what are known as “tail bits.” The bit stream thus output by device 1 is fed to a channel coder 2 where redundant bits are added to the information bits depending on the type of channel coding applied. As such, with most coding schemes, what are known as systematic bits are created on one side and parity bits on the other side. Depending on the coding rate of the channel coder 2, a greater or lesser number of systematic bits or parity bits are produced. In a few coding schemes, the parity bits are given lower priority or importance for decoding the corresponding message than the systematic bits. In UMTS mobile radio systems, channel coder 2 can, for example, be what is known as a turbo coder, which as a rule is constructed of nested folding coders.
A bit rate matching device 3 is connected downstream of channel coder 2 which punctures and/or repeats the bits fed to it in accordance with a specific bit rate matching algorithm. Because of the lesser importance or priority of the parity bits, the parity bits are preferably punctured for bit rate matching, since these are of less importance for successful decoding of the relevant message on the receiver side than the systematic bits.
The bit stream output by the bit rate matching device 3 is scrambled with the aid of an interleaver 4, so that the timing order of the individual bits is changed in accordance with a specific interleaving scheme. The result of processing by the interleaver 4 is that, for the bit stream outputted, the priorities of the individual bits are no longer known.
The bits outputted by the interleaver 4 are fed to a modulator 5 which, depending on the type of modulation used, maps several of these bits to specific symbols of a multidimensional symbol space and transmits the symbols to a receiver. With QPSK (“Quadrature Phase Shift Keying”) modulation, two bits are distributed in each case over four symbols equally spaced in a two-dimensional symbol space; whereas with 8PSK modulation three bits, are distributed with 16QAM (Quadrature Amplitude Modulation) four bits are distributed, and with 64QAM modulation six bits are assigned to a symbol in a two-dimensional symbol space.
The symbols generated by modulator 5 are transmitted in the form of a real and an imaginary part, which uniquely describe the position of the relevant symbol in the two-dimensional symbol space. A demultiplexer 7 connected downstream of modulator 5 distributes the systems possibly over a number of channels, where the sequence of symbols is encoded with different channelization or spread codes W1 . . . WM, which is shown in FIG. 1 in the form of a corresponding multiplexer 8. The sum signal of the different channel-coded symbol sequences is generated and output via a summator 9.
In addition, in accordance with FIG. 1, provision is made for a control unit 6 operating in accordance with AMCS (“Adaptive Modulation and Coding Schemes”), which defines the modulation alphabet of the modulator 5, as well as the encoding schemes and code rates of channel coder 2 and the distribution between the individual channelizing codes by the demultiplexer 7 to be used.
The transmit path structure shown in FIG. 1 corresponds, for example to the structure of the physical layer provided for what is known as HSDPA (“High Speed Downlink Packet Access”) in UMTS mobile radio systems. This involves a packet-switched connection type, in which case what is known as an ARQ (“Automatic Repeat Request”) method also can be used, in which case the receiver (for example, a mobile station) of a data packet, if this data packet is not received correctly, requests a repeat transmission of the packet by the transmitter (for example, a base station), after which the transmitter sends a repetition of the originally sent data packet to the receiver.
A problem with the function of the modulator shown in FIG. 1 is that, because of the type of modulation selected in each case, not all the bits directed to modulator 5 can be transmitted with equal security; (i.e., the reliability of the individual bits fluctuates, depending, for example, on the position of the symbol in the symbol space to which the individual bits will be mapped).
This will be explained in more detail below with reference to FIG. 6, which shows an example of the signal constellation of the two-dimensional symbol space 12 for a 16QAM modulation. In this diagram, four bits i1, q1, i2 and q2 are assigned in the specified order to a symbol 13 of the two-dimensional symbol space 12 shown in FIG. 6, in which case the type of mapping of the individual bits to the symbols 13 is referred to as “gray mapping.” In FIG. 6, the columns or rows of symbols are marked with a dash, in each case, which corresponds to a bit i1 or i2 or q1 or q2 with the value “1.” It can be seen from the diagram that, for example, the symbols with i2 “1” each have eight neighbors with the value i2=“0,” whereas symbols with i1=“1” only have four potential neighbors with i1=“0” and, thus, only four direct decision thresholds. The result of this is that the symbols with the bits i1=“1” are better protected from incorrect transmission than symbols with i2=“1.” The same also applies, for example to symbols q1=“1,” which have greater reliability than symbols with q2=“1.” As such, basically, in the signal constellation shown in FIG. 6 the bits i1 and q1 exhibit greater reliability with regard to correctly determining the information content than bits i2 or q2.
For the transmission path structure shown in FIG. 1, the problem which then arises is that on one side bits are provided with different priority or importance for decoding the relevant message, and on the other side the modulator 5 does not transmit bits equally securely or cannot map them to symbols equally reliably. Such is then transferred in the form of a real section Re or its inphase components and its imaginary part or their quadrature components so that, if necessary, bits with higher priority are mapped to symbols with lower reliability and transmitted. This makes the data transmission security and data transmission quality suffer.
In this connection, it already has been suggested for an attempted transmission of a data block that the bits are assigned in a specific way to the symbols 13 of symbol area 12, so that with a skillful application of an assignment specification, matching of the reliability of the individual bits can be achieved after several transmissions. However, this only applies if a data block is repeated several times. The transmission security is not improved by this suggestion for the first transmission of a data block. In addition, with this proposal the different priorities of the channel coder bits are not taken into account. A further problem associated with this suggestion lies in the fact that the repetition data packet does not absolutely have to be identical to the originally transmitted data packet because of the different mapping of the individual bits to the transmitted symbols. The result of this is that on the receiver side the two packet retransmissions can be combined directly before the demodulator, but what is known as a log likelihood combination must be undertaken at bit level. In this case, the received quadrature values are initially converted into the likelihoods for the transmitted bits in order to derive the actually transmitted bits with the greatest possible likelihood. However, the log likelihood combination does not perform as well as the previously mentioned simple symbol combination in which the symbols of the original data packet, weighted with the symbols of the retransmission data packet, are simply added before the demodulator after application of a channel estimation to the relevant signal-to-noise ratio, particularly with bad transmission characteristics. In addition, with the conventional combination of symbols, two items of bit information can be stored in an assigned memory location (for example, of the Inphase component) so that memory space can be saved.
A further proposal is shown in FIG. 2, whereby, after channel coder or turbo coder 2, which output the bits separately as systematic bits S and parity bits P, the systematic bits and parity bits will be processed separately. Therefore, two separate interleavers 4a and 4b are provided in which case, in one device 10, a parallel/serial conversion to just one bit stream takes place so that as intelligent an assignment as possible of the bits with different priorities or importances to the bit positions with differing reliability can be undertaken within the individual symbols. In this case, the bits with the highest priority; (i.e., the systematic bits S), are preferably distributed to the bit positions with the highest reliability and the bits with the lowest priority; (i.e., the parity bits), to the bit positions with the lowest reliability. Since frequently more bits with highest priority than bit positions with highest reliability are present, no optimum solution is possible as a rule. In addition, because of the multiplicity of interleavers and the additional device 10, this variant requires significantly more effort to implement.
An object of the present invention is, therefore, to provide a method for matching the bit rate of a bit stream to be transmitted in a communication system, as well as an associated communication device, in which case the data transmission quality and data transmission security can be improved with the minimum possible effort. In particular, the simplest possible methods and devices should be able to be used to guarantee that the more important bits are mapped to bit positions with high reliability within the individual modulation symbols.